1. Field of the Invention
The present invention relates to an optical scanning apparatus, and more particularly, to an optical scanning apparatus used for an image forming apparatus such as a laser beam printer (LBP), a digital copying machine and a multifunction printer (MFP).
2. Description of the Related Art
Conventionally, in an optical scanning apparatus included in an image forming apparatus such as a laser beam printer (LBP) or a digital copying machine, an optically-modulated beam is emitted from a light source such as a laser light source in response to an image signal. The optically-modulated beam is then periodically deflected by a deflector such as a polygon mirror. The deflected beam is condensed in a spot on a photosensitive surface of a photosensitive member serving as an image bearing member by an imaging optical system having an fθ characteristic. Accordingly, the photosensitive surface is optically scanned to perform image recoding.
Further, a tandem-type image forming apparatus has been proposed, in which a plurality of surfaces to be scanned respectively corresponding to each color in color image formation are simultaneously scanned. In an optical scanning apparatus included in the tandem-type image forming apparatus (hereinafter referred to as a “tandem-type optical scanning apparatus”), a plurality of beams deflected for scanning by the deflector are guided to a plurality of surfaces to be scanned that differ from each other, respectively, via corresponding imaging optical elements and reflection optical elements. The beam traveled through the imaging optical element and the reflection optical element scans a photosensitive surface as the surface to be scanned at a substantially constant speed.
However, there is a problem that a change of a level of an optical intensity (ratio of image plane illuminances) is generated depending on each image height. The ratio of image plane illuminances is generated due to a difference in transmittance of an optical element such as an imaging optical element, through which the beam emitted from the light source passes within a period until arriving at the surface to be scanned, depending on an incident angle of the beam, a difference in thickness of the imaging optical element depending on the image height, a defect of the fθ characteristic, or the like.
In addition, in the tandem-type optical scanning apparatus, it is necessary to equalize optical path lengths along which the plurality of beams deflected by the deflector arrive at the corresponding surface to be scanned, respectively. For this reason, considering an arrangement of each member, the number of reflection optical elements (the number of reflection surfaces) arranged on the optical path is set different between the plurality of optical paths in many cases. In addition, the reflection optical elements used on the respective optical paths generally have the same reflectance, and hence a difference in the ratio of image plane illuminances is generated on the surface to be scanned between the optical paths having the different numbers of reflection optical elements from each other. The ratio of image plane illuminances affects a density of a formed image in a main scanning direction, and hence in the tandem-type optical scanning apparatus, it is necessary to suppress the difference in the ratio of image plane illuminances between different optical paths.
Japanese Patent Application Laid-Open No. 2000-351234 discloses an apparatus which includes a light quantity measuring unit that measures a ratio of image plane illuminances on a surface to be scanned and a light intensity correction unit that corrects an intensity of a beam emitted from a light source, and corrects the ratio of image plane illuminances electrically based on a measurement result of the ratio of image plane illuminances. Further, Japanese Patent Application Laid-Open No. 2009-271352 discloses an apparatus which corrects a ratio of image plane illuminances by setting different reflectances for respective reflection optical elements and adding an angle dependency of the reflectance.
However, in the apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-351234, an additional unit such as the light intensity correction unit needs to be separately provided, and hence a configuration of the optical scanning apparatus becomes complicated. In addition, in the apparatus disclosed in Japanese Patent Application Laid-Open No. 2009-271352, a reflection optical element having a different angle dependency of the reflectance needs to be provided for each optical path in order to correct the ratio of image plane illuminances on each optical path. Therefore, the methods of correcting the ratio of image plane illuminances described in Japanese Patent Application Laid-Open No. 2000-351234 and Japanese Patent Application Laid-Open No. 2009-271352 still have a problem that the manufacturing cost of the optical scanning apparatus increases.